1. Field of the Invention
The present invention relates to a plate type heat exchanger and more particularly it relates to a plate type heat exchanger having a plurality of plates put together so as to define clearances therebetween through which two fluids pass for heat exchange.
2. Description of the Prior Art
A conventional plate type heat exchanger is constructed generally in the manner shown in FIGS. 1 through 3, wherein heat exchange plates are designated at 2 and 3 and there are provided packings 1a, 1a each extending around the outer periphery of such plate so as to define the peripheral edge of a fluid channel when said plates are put together. Each plate 2, 3 has four fluid passage holes at the four corners. There are distributing surfaces 22 and 32 between the fluid passage holes 21, 31 and heat exchange surfaces 23, 33. Designated at 2a and 3a are packing grooves extending around the peripheries of the passage holes 21 and 31, and a packing 1b is fitted in such groove in every other plate. Of the two fluids m and n on the heat transmitting and heat transmitted sides, one fluid, for example, m is allowed to flow into alternate fluid channels defined between the plates.
Since each packing 1b is disposed in every other spacing defined between the plates put together and clamped, the packing mount periphery lacks strength and tends to flex. As a countermeasure, it has previously been proposed to provide the distributing surface 22 of the plate 2 on the packing mount side with an intermediate step 2g, as shown in FIG. 4, and reduce the distance from a point of contact 4 between its bottom and an adjacent plate 3 to the packing groove. With such construction, however, when the plates are clamped although the strength of the plate 2 on the packing mount side is increased, the strength of the plate 3 on which such packing is not mounted remains insufficient. Rather, in the plate 3, the packing groove 3 is pushed by the packing 1b of the adjacent plate to flex as shown in phantom line, such flexure extending to the distributing surface 32, resulting in the latter flexing in the direction opposite to that of the flexure of the packing groove 3a (as ascertained by experimental results).
The packing 1b described above is disposed in the fluid passage hole region in the plate clearance, i.e., it is disposed between the passage hole and the distributing surface and serves to define the fluid passageway and strengthen the support for the plate clearance, but usually a double seal construction is employed as a seal construction for the peripheries of the fluid entrance and exit. For example, in a conventional embodiment shown in FIGS. 1 through 3, there is provided a double seal construction comprising said packing 1b disposed between the passage hole 21 and the distributing surface 22 and additionally a packing 1c disposed between the distributing surface 22 and heat exchange surface 23.
Conventionally, in such double seal construction, the packing groove width is the same for the packing mount side and no-packing mount side and hence there has been the danger that upon clamping of the plates the bottom surface of the packing groove 3j on the no-packing mount side is pressed by the packing 1c of the adjacent 2 to deform as shown in phantom line in FIG. 3. Such deformation not only greatly detracts from the sealing quality but also decreases the reinforcing property of the plate. As a countermeasure, as shown in FIG. 5, it is known to put the plate together with a reinforcing strip 5 received in the packing groove 3j on the no-packing mount side. The presence of such strip 5 improves the sealing quality but has the following disadvantages: The single strip adds to the number of assembling steps and hence to the cost. Further, the attachment of the strip 5 requires a thermal operation such as welding, so that the corrosion resistance of the welded portion is decreased. The presence of the strip 5 increases the resistance to the flow of fluid and can cause contamination.